Legend:

The problem is completely reproducible. I am running a BOINC (Berkeley distributed computing) app in the VM. BOINC has a built-in CPU benchmark. When ACPI is disabled, the benchmark shows performance of ~1800 MFlops/5500 MIPs. When ACPI is enabled, it shows ~500 MFlops/1400 MIPs. The actual app shows a similar change in speed.